This invention relates to an organic electroluminescent (EL) device and more particularly, to a structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
Active research works have been made on organic EL devices for use as displays because EL devices can be formed on a large area of glass. In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of tin-doped indium oxide (ITO) etc., a hole transporting layer of an organic amine compound, a light emitting layer of an organic luminescent material exhibiting electron conductivity and intense light emission such as an aluminum quinolinol complex (Alq3), and an electrode of a metal having a low work function such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. Structures having two or three organic compound layers are typical.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 3 and 4 illustrate typical configurations of organic EL devices.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15, both of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16, all of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Reliability is a common problem to be solved for these organic EL devices. More particularly, organic EL devices in principle have a hole injecting electrode and an electron injecting electrode and need a light emitting layer of organic material or organic layers for effectively injecting and transporting holes and electrons from the electrodes, respectively. However, the organic materials of which the organic layers are formed are vulnerable during manufacture and have less affinity to the electrodes. Another problem is raised by the significantly accelerated degradation of organic thin films as compared with inorganic thin films as in light emitting diodes (LED) and laser diodes (LD).
As the material of which the electron injecting electrode of the organic EL device is made, metals, metal oxides, and metal halides (typically metal fluorides) are promising because of their electron injecting efficiency. When the electron injecting electrode is formed by sputtering, its adhesion to the underlying organic layer is improved. However, the sputtering technique can do more damages to the organic layer than evaporation and other deposition techniques, causing development of non-light-emitting regions, known as dark spots, and current leakage. It also raises such problems as an increased initial drive voltage and an increased emission start voltage (or threshold voltage).
An object of the present invention is to provide an organic EL device which can be manufactured without damages to the organic layer and has minimized development of dark spots, a low initial drive voltage, a low emission start voltage, a high efficiency, and a long lifetime.
As disclosed in WO 98/24272 (International Application PCT/JP 97/04407, U.S. Ser. No. 08/984,087, EP 97 946073.0), Japanese patent application No. 9-128001 (U.S. Ser. No. 09/066,775, EP 98 303309.3) and Japanese Patent Application No. 9-145808 (U.S. Ser. No. 09/079,286, EP 98 303833.2), we found that organic EL devices having improved characteristics are obtained when an electron injecting electrode is made of a metal material, especially Alxe2x80x94Li alloy. However, when a metallic electron injecting electrode is used, it is difficult to find a trade-off between the adhesion and the damage of the electrode to the underlying organic layer. This problem must be solved before an organic EL device of better performance can be realized.
We have found that the problem can be solved by forming a layer of a porphyrin compound such as phthalocyanine or a naphthacene compound as a buffer layer between the electron injecting layer and the organic layer.
It is noted that Appl. Phys. Lett., Vol. 70, No. 22, 2 June 1997, xe2x80x9cA surface-emitting vacuum-deposited organic light emitting device,xe2x80x9d V. Bulovic, P. Tian, P. E. Burrows, M. R. Gokhale and S. R. Forrest and Appl. Phys. Lett., Vol. 72, No. 17, 27 April 1998, xe2x80x9cA metal-free cathode for organic semiconductor devices,xe2x80x9d G. Parthasarathy, P. E. Burrows, V. Khalfin, V. G. Kozlov and S. R. Forrest report a device (OILED) having 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) or copper phthalocyanine (CuPc) deposited between a hole injecting electrode of ITO and an organic layer of TPD and a device (TOLED) having copper phthalocyanine (CuPc) deposited between an electron injecting electrode of ITO and an organic layer of Alq3, respectively. It is described that the layer of copper phthalocyanine or the like has the function of protecting the organic layer from being damaged during deposition of ITO.
The devices reported in the literature, however, use ITO as the hole injecting electrode or electron injecting electrode. No study was made on the use of metals, metal oxides or metal halides as the electron injecting electrode. According to our experiment, a device using ITO as the electron injecting electrode is difficult to achieve practically acceptable performance because of an extremely low electron injecting efficiency. Therefore, the devices reported in the literature and the organic EL device using an electron injecting electrode of metal material according to the invention are completely different in function and effect, and the contents in the literature do not constitute the prior teachings from which the present invention is conceivable.
In a first aspect, the invention provides an organic electroluminescent (EL) device comprising a substrate, a hole injecting electrode and an electron injecting electrode on the substrate, an organic layer participating in at least a light emitting function disposed between the electrodes, and a buffer layer disposed between the organic layer and the electron injecting electrode. The electron injecting electrode is constructed of a metal, metal oxide or metal halide, and the buffer layer is constructed of a porphyrin or naphthacene compound.
In a second aspect, the invention provides a method for preparing an organic EL device comprising the steps of forming a hole injecting electrode on a substrate, forming thereon at least one organic layer participating in at least a light emitting function, forming thereon a buffer layer comprising a porphyrin or naphthacene compound, and forming thereon an electron injecting electrode.
Preferably, the electron injecting electrode is formed by sputtering and composed of an aluminum-lithium alloy containing 0.4 to 14 at % of lithium, more preferably 6.5 to 14 at % of lithium. The buffer layer typically has a thickness of 0.1 to 20 nm.